At the current stage, the Third Generation Partnership Project (3GPP) is discussing the RLC PDU re-segmentation related topics. Among others, one key issue is how to indicate the position of each segment in its original RLC PDU, so as to guarantee the receiver can order each segment correctly and assemble them into the original RLC PDU.
3GPP has determined that RLC PDU re-segmentation must support the following requirements:                The number of re-segmentation is unlimited;        The RLC PDU segment can be re-segmented; and        The RLC PDU header overhead for re-segmentation indication should be as low as possible.        
Some companies have commented that the number of re-segmentation will not be very high in real situation, for example, 2 or 3 maybe the major case. However, we cannot exclude the higher-numbered re-segmentation scenario, which maybe rare cases.
Two solutions on how to indicate each RLC PDU segments have been proposed in the prior art:                Solution 1: Offset;        Solution 2: Segment index which only supports a limited number of re-segmentation.        
These two solutions are described respectively below.                Solution 1: Offset        
In the solution, a 16-bit field “Offset” is added in each RLC-PDU segment to indicate its first byte's position in the original RLC PDU. The 16-bit field is determined according to a peak rate of 100 Mbit in LTE (Long Term Evolution) DL (downlink).
FIG. 1 is a schematic diagram for explaining the Offset solution.
As shown in FIG. 1, it is assumed that the original RLC PDU is segmented into 3 parts, including RLC PDU segments 1-3. The RLC PDU segment 2 is further segmented into RLC PDU segments 4 and 5 when it is re-transmitted. That means, the receiver finally receives RLC PDU segments 1, 3, 4, 5. In each segment, one field “Offset” with a 16-bit fixed size is used to indicate the start position of each segment in the original RLC PDU.                In RLC PDU segment 1, the Offset indicates its first byte's position is A in the original RLC PDU;        In RLC PDU segment 2, the Offset indicates its first byte's position is B in the original RLC PDU;        In RLC PDU segment 3, the Offset indicates its first byte's position is D in the original RLC PDU;        In RLC PDU segment 4, the Offset indicates its first byte's position is B in the original RLC PDU; and        In RLC PDU segment 5, the Offset indicates its first byte's position is C the original RLC PDU.        
It is obvious that the receiver (Rx) can order each segment correctly in an order of segment 1, segment 4, segment 5 and segment 3, according to their “Offset” fields.
The bits of the Offset fields transmitted in the Uu interface are calculated as follows:                When the RLC PDU segment 1 is transmitted, a 16-bit Offset field is sent in the Uu interface;        When the RLC PDU segment 2 is transmitted, a 16-bit Offset field is sent in the Uu interface;        When the RLC PDU segment 3 is transmitted, a 16-bit Offset field is sent in the Uu interface;        When the RLC PDU segment 4 is transmitted, a 16-bit Offset field is sent in the Uu interface;        When the RLC PDU segment 5 is transmitted, a 16-bit Offset field is sent in the Uu interface.        
Then totally 16*5=80 bits are transmitted when the Rx has received all the segments.
On one hand, the solution has the advantages as follows:                The unlimited re-segmentation is supported; and        The segment can be re-segmented.        
On the other hand, the solution has the drawbacks as follows:                The 16-bit Offset field leads to a big RLC PDU header overhead; and        Some companies prefer to use the segment index solution.        Solution 2: Segment Index        
In this solution, a fix-sized Sub-segment Index (SegInd) is added in each segment to indicate its order in the original RLC PDU.
FIG. 2 is a schematic diagram for explaining the Segment Index solution.
As shown in FIG. 2, a 4-bit “SegInd” field is attached in each segment to show its order in the original RLC PDU. It is clear that the original RLC PDU can only be segmented into for example 24=16 blocks.
For example, 0000, 0001 and 0002 are set for segments 1, 2, 3 respectively, as respective segment indexes. However, Re-segmentation of segment 2 into segments 4 and 5 can not be supported in this solution, because the segment index SegInx cannot be set correctly in such case.
On one hand, this solution has the advantages as follows:                Less bits used for the segment index SegInx compared to the 16-bit Offset field help to decrease the segment header overhead, especially when the support segment number is small, which maybe the major case in real situation.        
On the other hand, this solution has the drawbacks as follows:                The un-limited re-segmentation can not be supported; and        The further re-segment of a segment can not be supported.        
Therefore, one solution is needed which introduces small re-segmentation header overhead at low re-segmentation level, while is adaptive for supporting un-limited high re-segmentation level. Such solution is proposed according to the present invention to satisfy such requirement.